The aromatic hydrocarbons (specifically benzene, toluene and xylenes), are the main high-octane bearing components of the gasoline pool and important petrochemicals used as building blocks to produce high value chemicals and a variety of consumer products, for example, styrene, phenol, polymers, plastics, medicines, and others. Aromatics are primarily produced from oil-derived refinery feedstocks in such processes as catalytic reforming and cracking of heavy naphthas. However, the recent severe oil shortages and price spikes resulted in severe aromatics shortages and price spikes. Therefore, there is a need to develop new commercial routes to produce high value aromatics from highly abundant and cheap hydrocarbon feedstocks, for example, methane or stranded natural gas (typically containing about 80-90% methane).
There are enormous proven stranded natural gas reserves around the world. According to some estimates, the natural gas reserves are at least equal to those of oil. However, unlike the oil reserves which are primarily concentrated in a few oil-rich countries and are properly and extensively exploited, upgraded and monetized, the natural gas reserves are much more broadly distributed around the world and significantly underutilized. Many developing countries that have significant natural gas reserves lack the proper infrastructure to exploit them and convert them to higher value products. Quite often, in such situations, natural gas is flared to the atmosphere and wasted. Because of the above reasons, there is enormous economic incentive to develop new technologies that can efficiently convert methane or natural gas to higher value chemical products, specifically aromatics.
In 1993, Wang et al., (Catal. Lett. 1993, 21, 35-41), discovered that methane can be partially converted to benzene at atmospheric pressure and a temperature of 700° C. over a catalyst containing 2.0 wt % molybdenum deposited on H-ZSM-5 zeolite support. Significantly, low methane conversion of less than 10% but very high benzene selectivity of 100% were observed in these experiments. Subsequently, other researchers repeated the above work and found that Wang et al. did not quite identify all of the reaction products (naphthalene and others) and that when all of the products are identified the benzene selectivity falls in the range of 60-70%. These other researchers also pointed out that, the catalyst cokes up and deactivates very rapidly—as manifested by complete loss of activity after about 4-5 hrs on stream. Since Wang's discovery, many academic and industrial research groups have contributed to further developing various aspects of the methane to benzene catalyst and process technology. Many catalyst formulations have been prepared and tested and various reactor and process conditions and schemes have been explored.
Despite these efforts, there is still no commercial methane aromatization or methane to benzene catalyst and process. The vast majority of researchers agree that the main obstacles to developing and commercializing an efficient, direct methane to benzene process are the low methane conversion (still remaining at around 7-10%) and rapid coke formation and catalyst deactivation.
Therefore, there is a need to develop new methane aromatization catalysts that provide higher methane conversion at equal or higher selectivity to benzene relative to the prior art. Also, there is a need to develop catalysts that exhibit lower coking and deactivation rates, i.e. better sustain their methane conversion and benzene selectivity performance over the course of time (exhibit better stability) relative to the catalysts of the prior art.